An investigation of in-situ optical monitoring during metalorganic vapour phase epitaxy (MOVPE) growth of GaN is reported. We describe the modelling of experimental optical data to determine the important properties of the GaN films such as the high temperature optical constants, the evolving surface roughness during coalescence and the film growth rate. Real time spectral reflectometry has been implemented as a routine tool for quantitative and qualitative monitoring during GaN MOVPE growth across the wavelength range 300-750 mn. The technique allows rapid material optimisation for device performance and tracking of run to run variation. Two multilayer optical modelling techniques have been investigated to simulate the time-dependent reflectance signatures. A virtual interface method where the simulation must begin after a smooth surface has been achieved, and an enhanced modelling routine, developed in-house, to include the evolving surface roughness during coalescence. It is shown that the high temperature optical constants are in close agreement with the limited published literature values, and the growth rate is consistent with post-growth thickness measurements. Using atomic force microscopy (AFM) and spectroscopic ellipsometry to study the surfaces of interrupted GaN growth runs, we show for the first time that the surface topography of a GaN film during coalescence contains components of both macroscopic and microscopic roughness with respect to the optical monitoring wavelengths. Additionally, we obtain very good agreement between values of RMS roughness determined by in-situ spectral reflectometry and spectroscopic ellipsometry compared with AFM. (C) 2002 Elsevier Science B.V. All rights reserved.